A key is most typically an elongated, rectangular cross-section member positioned between a rotatable shaft and a device rotating with the shaft. A portion of the cross-section of the key is seated in a keyway disposed axially in the shaft and a portion of the cross-section is seated in a keyway in the device facing the shaft keyway. The key functions to transfer torque between the shaft and the device. Common types of devices which rotate with the shaft include sprockets, gears, pulleys and vehicle wheels. In apparatus comprising these types of devices, either the device or the shaft may be the driving member. A further use of keys, usually in conjunction with their torque transfer function, is for providing guidance when positioning a device axially on a shaft. Keys may be made of a variety of materials including steel, other metals, polymeric, and reinforced polymeric materials.
Various types of mechanisms utilize the principle of a key seated in a keyway for transferring rotating force from one rotating member to another. In these mechanisms, the key is not necessarily rectangular in cross-section and the overall mechanism of which the key is a part may perform functions in addition to rotational force transfer. Also, the force transfer may not be from one keyway to another through the key but, for example, may be through a shaft keyway and the key to a portion of the mechanism integral with the key and bolted to a driven member.
In virtually all types of keys used for rotational force transfer, the stresses induced in the key are substantial. Such stresses frequently lead to failure of the key and/or the driven or driving member. Consequently, keys as well as the members they are seated in are preferably made of a high compressive strength material such as steel, one of the materials previously mentioned. However, where light weight and lower cost materials and drive systems are desired, steel may not be the preferred choice for the keys and the associated drive members. Polymeric materials such as nylon, acetal and polyethylene are quite popular in lower cost drive systems. However, driven and driving members as well as keys made of these materials are somewhat lacking in physical strength properties and fail more quickly than such members made of steel. For example, where the key cross-section is rectangular and a portion of the crosssection is seated in a keyway in a driving member and the balance of the cross-section is seated in a keyway in a driven member, the key is subject to failure due to torsional force about the axis of the keyway which causes the key to fail by rotation within the facing keyways of the driven and driving members. Part of this failure will involve deformation or disintegration of the keyway walls or the edges of the key, or both.